As a method for the alkylation of an aromatic compound, a Friedel-Crafts alkylation reaction or a gaseous phase reaction which uses a solid acid catalyst are widely known.
In alkylating an aromatic compound, the position (ortho, meta or para position) of the aromatic ring of the raw material to which an alkyl group is introduced is determined by the effect of the functional group (orientation). When the desired compound is not in accord with the substitution orientation inherent to the functional group, however, a positional isomer of the desired compound is obtained as a product. Therefore, it is necessary to carry out disproportionation called an isomerization reaction or a transalkylation reaction in order to introduce the alkyl group to the desired position.
For example, the Friedel-Crafts alkylation is a method known from the past, in which a Lewis acid catalyst such as aluminum chloride is used as a catalyst. With a method using aluminum chloride as an alkylation agent, the selectivity to a monoalkyl compound in the alkylation reaction is low. Further, the method requires complicated steps of separation of the monoalkylated product from a polyalkylated product by distillation and the subsequent transalkylation of the polyalkyl compound to the monoalkyl compound (JP-A-Sho 57-40419).
JP-A-Hei 04-346939 discloses a method in which alkylation is carried out using aluminum chloride, followed by transalkylation using a solid acid catalyst. This method, however, requires removal or separation by distillation of aluminum chloride and HCl used as a co-catalyst prior to the transalkylation and, therefore, is a complicated multi-step process.
U.S. Pat. No. 5,030,777 discloses a process for producing 3,5-dichloroalkylbenzene in which a dichlorobenzene is subjected to an alkylation reaction using aluminum chloride as a catalyst and isopropyl bromide as an alkylating agent, followed by isomeriziation and transalkylation. This process has problems that a halogenated alkyl must be used as a raw material and that the process steps are complicated.
The Friedel-Crafts alkylation reaction using aluminum chloride requires complicated steps as described above. Further, the amount of the catalyst relative to the raw material is large. Furthermore, the catalyst is apt to be inactivated because the catalyst tends to form a complex with various compounds produced by the reaction. Additionally, in order to separate the product from aluminum chloride after termination of the reaction, it is necessary to treat the reaction mixture with water. As a consequence of the treatment, aluminum chloride turns into aluminum hydroxide. Thus, the method has a defect that it is difficult to recycle the catalyst.
With regard to the alkylation reaction using other Lewis acids, U.S. Pat. No. 4,943,668 discloses an alkylation reaction of metaxylene using aluminum halide and iodine as a catalyst and an α-olefin as an alkylating agent. This method, however, has defects that iodine should be used in addition to aluminum halide and that the reaction time is as long as 2 to 7 hours. U.S. Pat. No. 4,048,248 discloses a method of alkylating an aromatic compound using titanium tetrafluoride. With this method, the selectivity to the desired product is 60 to 70% and is not satisfactory.
JP-A-Hei 06-263656 discloses a method in which a rare earth-containing catalyst using a perfluoroalkyl group-containing sulfonyl group as a counter anion is used. This method is superior in comparison with a method using an aluminum halide catalyst, since the catalyst may be recycled. However, this method has problems that the catalyst is expensive and that the selectivity is low due to accompanying polyalkylation reaction. Namely, the yield of mono-substituted products is about 30%, the yield of di-substituted products is about 10 to 30%, and the yield of tri-substituted products is about 10%. U.S. Pat. No. 4,158,677 discloses an example of dialkylating an alkylbenzene using a carboxylic acid complex of BF3 for the production of synthetic oils (lubricants). While the reaction gives a yield of the desired product of as high as about 90%, this method has problems that a long reaction time of 20 hours or more is required and that the post treatment of the reaction liquid is troublesome.
As described above, any conventional alkylation reaction using a Lewis acid as a catalyst has problems. Further, as a common problem in all manufacturing methods, the reaction requires multi-steps and complicated post treatment and, therefore, is ill-suited for industrial production.
Described in the foregoing are alkylation reactions using as a catalyst a Lewis acid represented by aluminum chloride. Also well known is an alkylation reaction using as a catalyst HF which is a Broensted acid. Further, a reaction using HF as an alkylation catalyst is disclosed. This method pertains to a process for producing gasoline having an improved octane number, wherein C2 to C20 olefins containing HF and a paraffin are passed through a column having a fixed bed of an inert support. From the object thereof, the product must be a mixture of various compounds. Thus, this method is not considered to pertain to selective alkylation. For example, in the case of trimethylpentane which is the major product, the weight proportion is about 70% and the selectivity is low (U.S. Pat. No. 4,783,567 and No. 4,891,466).
U.S. Pat. No. 2,766,307, No. 2,803,682 and No. 2,803,683 and D. A. McCaulay and A. P. Lien, J. Am. Chem. Soc., 77, 1803 (1955) disclose a method for producing alkylxylenes by ethylation or isopropylation of metaxylene. In the methods disclosed in the above patents and article, the catalyst system uses HF and BF3 at the same time. Further, HF is used in an amount of 10 to 20 moles per mole of the raw material substituted aromatic compound, which is much greater than that in the present invention. Thus, a problem is caused that the efficiency of the separation and purification of HF used as the catalyst and the desired product is low.
As an alkylation method using other acids, there may be mentioned fixed bed alkylation using a fluorinated sulfonic acid catalyst and reaction using a zeolite catalyst (JP-A-Hei 09-2982). However, in the example using a fluorinated sulfonic acid catalyst, the conversion of the raw material aromatic compound is not satisfactory (about 30%). Further, the method has a problem with respect to the selectivity, since branched alkylated products and polyalkylated products are produced. A method disclosed in JP-A-2000-297049 and JP-A-2002-20325 uses zeolite catalysts. While the method is excellent with respect to easiness in separation of the product from the catalyst, polyalkylated products are by-produced in an amount of 10% or more. Additionally, the catalyst is expensive.